Method for manufacturing bainite high-strength seamless steel tube, and bainite high-strength seamless steel tube

Abstract

A method for manufacturing a bainite high-strength seamless steel tube, comprising the following steps: smelting, manufacturing a billet, heating, perforating, rolling, stretch reducing or sizing to obtain tube, and cooling. In the cooling step, the quenching starting temperature is controlled to be at least 20° C. higher than the Ar3 temperature of the steel grade; the finish cooling temperature is controlled to be within a range between T1 and T2, where T1=519-423 C-30.4Mn, T2=780-270 C-90Mn, and the units of the T1 and the T2 are ° C.; in the formulas, C and Mn respectively represent the mass percents of element C and element Mn of the steel grade, the content of the element C is 0.06-0.2%, and the content of the element Mn is 1-2.5%; the cooling rate is controlled to be 15-80° C./s; and the finished product of the bainite high-strength seamless steel tube is directly obtained after the cooling step. The manufacturing of a bainite high-strength seamless steel tube using the method requires neither the addition of precious alloying elements nor the subsequent heat treatment. Therefore the production costs are low.

Claims

1. A method for manufacturing a bainite seamless steel tube consisting of chemical elements by mass: C, 0.06-0.2%; Si, 0.1-0.6%; Mn, 1-2.5%; Al, 0.01-0.1%; S≤0.005%; P≤0.02%; O≤0.01%, and the balance being Fe and other unavoidable impurities, wherein the mass percentages of the element C and the element Mn satisfy: C+Mn/6≥0.38, the method comprising the following ordered steps: smelting; manufacturing a billet; heating the billet; piercing, rolling and stretch reducing or sizing to obtain a tube; cooling the tube, wherein the cooling step comprises a cooling rate of 15-80° C./s and commences once the tube reaches a temperature greater than or equal to the Ar3 temperature of the steel grade +20° C. and ceases once the tube achieves a temperature within a range between T1 and T2, wherein T1=519-423 C-30.4Mn, T2=780-270 C-90Mn, and units of T1 and T2 are ° C. in the formulas, wherein C and Mn respectively represent the mass percent of element C and element Mn of the steel grade; obtaining a bainite seamless steel tube, wherein the bainite seamless steel tube is directly obtained after the cooling step.

2. The method for manufacturing a bainite seamless steel tube according to claim 1, wherein the cooling step comprises water cooling.

3. The method for manufacturing a bainite seamless steel tube according to claim 2, wherein the water cooling comprises spraying water on the outer wall of the tube for cooling.

4. The method for manufacturing a bainite seamless steel tube according to claim 2, wherein the cooling step comprises placing the tube in a sink for cooling.

5. The method for manufacturing a bainite seamless steel tube according to claim 1, wherein the heating step comprises heating the billet to 1150-1300° C. and maintained for 1-4 hours.

6. The method for manufacturing a bainite seamless steel tube according to claim 1, wherein the bainite seamless steel tube manufactured by said method has a yield strength >555 MPa, and an impact energy (full size test piece) at 0° C. of >50 J.

Description

DETAILED DESCRIPTION

(1) The method for manufacturing a bainite high-strength seamless steel tube and the bainite high-strength seamless steel tube manufactured by the method are now explained and described accompanying the specific embodiments as follows, and the explanation and the description shall not be deemed to limit the technical scheme of the invention.

Example A1-A8 and Comparative Example B1-B7

(2) Bainite high-strength seamless steel tubes in Example A1-A8 and Comparative Example B1-B5 were manufactured according to the following steps:

(3) (1) smelting, and controlling steel composition as shown in Table 1 (it should be noted that the steel component of the smelting step is the same as that of the bainite high-strength seamless steel tube products);

(4) (2) manufacturing a billet: the smelted molten steel was directly cast into a round billet, or cast into blank followed by forging or rolling into a round billet;

(5) (3) heating: the round billet was heated to 1150-1300° C. and maintained for 1-4 hours;

(6) (4) piercing;

(7) (5) rolling;

(8) (6) stretch reducing or sizing to obtain tube;

(9) (7) cooling: the quenching starting temperature was controlled to be at least 20° C. higher than the Ar3 temperature of the steel grade; the finish cooling temperature was controlled to be within a range between T1 and T2, where T1=519-423 C %-30.4Mn %, T2=780-270 C %-90Mn %, and the units of the T1 and the T2 were ° C.; in the formulas, C and Mn respectively represented the mass percents of element C and element Mn of the steel grade, the content of the element C was 0.06-0.2%, and the content of the element Mn was 1-2.5%; the cooling rate was controlled to be 15-80° C./s; and the finished product of the bainite high-strength seamless steel tube was directly obtained after the cooling step (see Table 2 for the specific process parameters of each embodiment and comparative example).

(10) Table 1 lists the mass percentages of chemical elements of Example A1-A8 and Comparative Example B1-B7.

(11) TABLE-US-00001 TABLE 1 (by wt %, the balance is Fe and other impurities except O, P and S) Classi- Compositions ( wt %) C + fications No. C Si Mn P S O Al Mn/6 Examples A1 0.1  0.17 1.82 0.012 0.003 0.005 0.02  0.40 A2 0.18 0.36 1.25 0.018 0.003 0.004 0.015 0.39 A3 0.09 0.25 1.96 0.016 0.001 0.008 0.03  0.42 A4 0.18 0.38 1.78 0.012 0.002 0.003 0.07  0.48 A5 0.07 0.25 2.14 0.018 0.002 0.004 0.04  0.43 A6 0.15 0.58 1.65 0.016 0.004 0.005 0.02  0.43 A7 0.16 0.28 1.31 0.012 0.002 0.003 0.035 0.38 A8 0.14 0.35 1.49 0.018 0.002 0.002 0.03  0.39 Com- B1 0.13 0.18 1.73 0.008 0.02  0.42 parative B2 0.18 1.23 0.015 0.004 0.005 0.08  0.45 Examples B3 0.15 0.17 1.17 0.01  0.002 0.002 0.02  B4 0.14 0.35 1.49 0.018 0.002 0.002 0.033 0.39 B5 0.14 0.35 1.49 0.018 0.002 0.002 0.04  0.39 B6 0.14 0.35 1.49 0.018 0.002 0.002 0.03  0.39 B7 0.14 0.35 1.49 0.018 0.002 0.002 0.05  0.39

(12) It can be seen from Table 1 that the contents of P and S in Comparative Example B1 are higher than the preferred range of the present invention; the content of C in Comparative Example B2 is higher than the preferred range of the present invention; the value of C+Mn/6 in Comparative Example B3 does not match the preferred range of the present invention.

(13) Table 2 lists the specific parameters of the manufacturing methods of Example A1-A8 and Comparative Example B1-B7.

(14) TABLE-US-00002 TABLE 2 Cooling Heating T1 T2 Heating Quenching Finish (T1 = 519- (T2 = 780- Average tempera- starting cooling 423° C. 270° C. cooling ture/ Holding Cooling Ar3/ temper- temper- %-30.4 Mn %-90 Mn rate/ Classifications No. ° C. time/h mode.sup.note ° C. ature/° C. ature/° C. %)/° C. %)/° C. ° C./s Examples A1 1260 2 Immersing 814 860 480 421.37 589.2 45 A2 1240 2 Immersing 816 910 460 404.86 618.9 32 A3 1200 2 Spraying 817 960 500 421.35 579.3 23 A4 1300 2 Immersing 809 950 540 388.75 571.2 38 A5 1190 2 Immersing 818 840 520 424.33 568.5 40 A6 1260 2 Spraying 825 910 470 405.39 591   29 A7 1280 2 Spraying 815 860 500 411.50 618.9 27 A8 1270 2 Spraying 819 850 600 414.48 608.1 28 Comparative B1 1250 2 Immersing 810 920 510 411.42 589.2 34 Examples B2 1250 2 Immersing 798 910 500 380.09 604.5 33 B3 1260 2 Spraying 814 870 490 419.98 634.2 28 B4 1130 2 Spraying 819 490 414.48 608.1 30 B5 1290 2 Spraying 819 890 500 414.48 608.1 B6 1290 2 Spraying 819 890 414.48 608.1 24 B7 1290 2 Spraying 819 890 414.48 608.1 25 Note: cooling mode—spraying (spraying on the outer wall for cooling), immersing (immersing the tube into the sink for cooling)

(15) It can further be seen from Table 2 that the quenching starting temperature of Comparative Example B4 is lower than the range defined by the present invention, and the cooling rate of Comparative Example B5 is lower than the range defined by the present invention. The finish cooling temperature of Comparative Example B6 is higher than the range defined by the present invention and the finish cooling temperature of Comparative Example B7 is lower than the range defined by the present invention.

(16) Table 3 shows the measured parameters of mechanical properties of the seamless steel tubes of Example A1-A8 and Comparative Example B1-B7 placed on the cooling bed and air cooled to room temperature.

(17) TABLE-US-00003 TABLE 3 Yield Impact energy/J strength (full size test Classifications No. Rp0.2/MPa piece, 0° C. ) Examples A1 588 148 A2 725 127 A3 590 224 A4 672 93 A5 608 170 A6 696 109 A7 598 121 A8 614 107 Comparative B1 705 Examples B2 660 B3 68 B4 154 B5 165 B6 124 B7 815

(18) In Table 3 above, the performance test results are from the following tests:

(19) (1) Strength test: the prepared seamless steel tube is processed into an API arc sample, and the average value is obtained after the inspection according to the API standard to obtain the yield strength.

(20) (2) Impact toughness test: the prepared seamless steel tube is processed into a standard impact sample with 10*10*55 size and V-notch, which is tested at 0° C.

(21) As can be seen from Table 3, the yield strengths of the seamless steel tubes of Example A1-A8 are all higher than 550 MPa. and the impact energies (full size test piece) at 0° C. are all higher than 50 J, which is superior to the corresponding performances of Comparative Example B1-B7, and those seamless steel tubes have advantages of high strength and high toughness, which can be applied in oil and gas production, mechanical structure and other fields, meeting the corresponding mechanical performance indicators in this field. Meanwhile, the residual heat during the manufacture of seamless steel tubes is fully utilized, and the manufacturing process is convenient, basically not adding alloying elements, and the cost can be controlled in a lower range.

(22) It can also be seen from Table 3 that the impurity elements P and S of Comparative Example B1 exceed the optimized range, reducing the impact toughness of the seamless steel tube; the content of C of Comparative Example B2 is too high, so that the seamless steel tube influenced by both deformation stress and transformation stress during cooling are likely to crack, reducing the impact toughness; C+Mn/6<0.38 in B3 affects hardenability, and the deformation is insufficient, affecting the effect of the deformation inducing phase transition, reducing the yield strength; insufficient quenching starting temperature of Comparative Example B4 leads to the formation of the pro-eutectoid ferrite in the matrix structure, reducing the yield strength; the cooling rate of Comparative Example B5 is too low and it leads to insufficient proportion of martensite in the matrix structure, reducing the yield strength; the finish cooling temperature of Comparative Example B6 is too high to obtain the required bainite, reducing the yield strength; the finish cooling temperature of Comparative Example B7 is too low and it leads to excessive martensite, reducing the impact toughness.

(23) It should be noted that the above examples are only specific embodiments of the invention. Apparently, the invention is not limited to the above embodiments, and there may be many similar variations. A person skilled in the art can directly derive or associate all the variations from the content disclosed by the invention, all of which shall be covered by the protection scope of the invention.